1. Field of the Invention
The present invention relates to radio frequency identification (RF/ID) systems, and more particularly, to an RF/ID transponder using a dipole-over-ground plane antenna adapted to provide a radiation pattern including squinted beams that can be detected by an interrogator from a lateral orientation relative to the RF/ID transponder.
2. Description of Related Art
In the automatic data identification industry, the use of RF/ID transponders (also known as RF/ID tags) has grown in prominence as a way to track data regarding an object on which an RF/ID transponder is affixed. An RF/ID transponder generally includes a semiconductor memory in which information may be stored. An RF/ID interrogator containing a transmitter-receiver unit is used to query an RF/ID transponder that may be at a distance from the interrogator. The RF/ID transponder detects the interrogating signal and transmits a response signal containing encoded data back to the interrogator. RF/ID systems are used in applications such as inventory management, security access, personnel identification, factory automation, automotive toll debiting, and vehicle identification, to name just a few.
Such RF/ID systems provide certain advantages over conventional optical indicia recognition systems (e.g., bar code symbols). For example, the RF/ID transponders may have a memory capacity of several kilobytes or more, which is substantially greater than the maximum amount of data that may be contained in a bar code symbol. The RF/ID transponder memory may be re-written with new or additional data, which would not be possible with a printed bar code symbol. Moreover, RF/ID transponders may be readable at a distance without requiring a direct line-of-sight view by the interrogator, unlike bar code symbols that must be within a direct line-of-sight and which may be entirely unreadable if the symbol is obscured or damaged. An additional advantage of RF/ID systems is that several RF/ID transponders may be read by the interrogator at one time.
Often it is desirable to place an RF/ID transponder on a horizontal surface facing upward, such as on a cargo container or warehouse pallet, etc. In such applications, the radiation pattern emitted from the antenna of the RF/ID transponder extends normal to the horizontal plane, or straight upward. This orientation renders the RF/ID transponder difficult to detect from an interrogator that approaches the RF/ID transponder from the side, such as an interrogator that is mounted on a fork lift vehicle operating in a warehouse. In such an example, the interrogator will not detect the RF/ID transponder until it is literally right on top of the transponder. Since the effective reading range of the RF/ID transponder is reduced, the utility of the RF/ID system is substantially diminished. Consequently, it would be very desirable to develop an RF/ID transponder having an antenna radiation pattern that includes an angular region permitting detection by an interrogator approaching from the side.